Variable feedback circuits and methods

ABSTRACT

In one embodiment, the present invention includes a circuit comprising an amplifier having an input terminal, an output terminal, a positive supply voltage, and a negative supply voltage. The amplifier is configured to have a first gain. A variable feedback circuit is coupled between the input terminal and the output terminal. The difference between the voltage on the output terminal and input terminal is received by the variable feedback circuit, which changes the gain of the circuit and reduces distortion.

BACKGROUND

The present invention relates to circuits, and in particular, to avariable feedback circuit.

FIG. 1 is an example of a prior art amplifier circuit 100. Circuit 100in this example includes an amplifier 101 having first and second inputterminals (e.g., positive and negative inputs) and an output terminal.In this example, amplifier 101 includes a first resistor R1 coupledbetween an input signal Vin and the first negative input terminal. Asecond resistor R2 is coupled between the output terminal of theamplifier and the first input terminal. The second positive inputterminal is coupled to ground. This amplifier configuration is thecommonly-known inverting amplifier. The gain of this stage is given bythe following equation:Vout/Vin=−R2/R1.In this example, amplifier 101 is powered by a +5 volt positive supplyvoltage and a zero volt (i.e., ground) negative supply voltage. Thus, ifR2 is ten times the value of R1, then the circuit will have a gain often (10). Therefore, if the input signal has a peak-to-peak amplitude of600 mV, then the output, Vout, should be a signal having a peak-to-peakamplitude of 6V. This example highlights a limitation of non-idealcircuits. In particular, since the positive supply is only 5V, thevoltage on the output terminal, Vout, cannot achieve a voltage greaterthan 5V (typically less for amplifiers that do not have rail-to-railoutput swing capability). When the input is sufficiently large inamplitude to cause the voltage on the output to reach the upper and/orlower power supply voltages (i.e., the upper and lower rails), thevoltage on the output terminal will increase to the rail and then stop.This is referred to as “clipping.” A clipped output signal, Vout, isplotted to the right of the amplifier circuit in FIG. 1. As shown in theplot, the top and bottom of the signal are flat, indicating that theinput signal and gain have cause the voltage on the output to reach amaximum value as constrained by the upper and lower rails.

Clipping is problematic in many circuit applications because during theperiod of time that the signal is clipped, the information contained inthe input signal is lost. Clipping is particularly problematic inapplications that are sensitive to distortion. For example, in audioapplications it is desirable to minimize the amount of amplifierdistortion because even the slightest distortion may be discemable tothe human ear.

Thus, there is a need to reduce distortion in electronic circuits. Thepresent invention solves these and other problems by providing variablefeedback circuits and methods.

SUMMARY

Embodiments of the present invention reduce distortion effects incircuits. In one embodiment the present invention includes a circuitcomprising an amplifier having an input terminal, an output terminal, apositive supply voltage, and a negative supply voltage, the amplifierconfigured to have a first gain, and a variable feedback circuit coupledbetween the input terminal and the output terminal, wherein if thedifference between the voltage on the output terminal and the positivesupply voltage is less than a predetermined value, then the variablefeedback circuit reconfigures the amplifier to have a second gain lessthan the first gain, and if the difference between the voltage on theoutput terminal and the negative supply voltage is less than apredetermined value, then the variable feedback circuit reconfigures theamplifier to have a third gain less than the first gain.

In one embodiment, the variable feedback circuit comprises a comparisoncircuit.

In another embodiment, the comparison circuit compares the voltage onthe output terminal to the voltage on the input terminal and activates afirst current path between the output terminal and the input terminalwhen the voltage difference between the output terminal and inputterminal is positive and greater than a predetermined value.

In another embodiment, the comparison circuit compares the voltage onthe output terminal to the voltage on the input terminal and activates asecond current path between the output terminal and the input terminalwhen the voltage difference between the output terminal and inputterminal is negative and greater than a predetermined value.

In one particular embodiment, the variable feedback circuit comprises aresistor and at least one transistor.

In another embodiment, the variable feedback circuit comprises a firsttransistor having a first terminal coupled to the output terminal, asecond terminal coupled to the input terminal through a first resistor,and a control terminal coupled to a first reference voltage, and asecond transistor having a first terminal coupled to the outputterminal, a second terminal coupled to the input terminal through thefirst resistor, and a control terminal coupled to a second referencevoltage.

In one embodiment, the first and second transistors are bipolartransistors.

In another embodiment, the amplifier includes a third resistor having afirst terminal coupled to the input terminal and a second terminalcoupled to an input signal, and a fourth resistor coupled between theamplifier input terminal and output terminal, wherein the amplifierfurther comprises a second terminal coupled to ground, and wherein thefirst transistor is coupled to a first intermittent node in a pluralityof series connected resistors coupled between the positive supplyvoltage and the negative supply voltage, and the second transistor iscoupled to a second intermittent node in said plurality of seriesconnected resistors.

In one embodiment, the variable feedback circuit comprises means forcoupling a resistor between the output terminal and input terminal ifthe voltage difference between the output terminal and input terminal ispositive and greater than a first predetermined value and if the voltagedifference between the output terminal and input terminal is negativeand greater than a second predetermined value.

Alternatively, embodiments of the present invention include circuitcomprising an amplifier having an first and second input terminals, anoutput terminal, a positive supply voltage, and a negative supplyvoltage, a first resistor coupled between the output terminal and thefirst input terminal, a first transistor having a first terminal, asecond terminal, and a control terminal, wherein the first terminal ofthe first transistor is coupled to the output terminal of the amplifierand the control terminal is coupled to a first reference voltage, and asecond resistor coupled between the second terminal of the firsttransistor and the first input terminal of the amplifier. In oneembodiment, the first reference voltage is configured so that the firsttransistor conducts current between the output terminal of the amplifierand the second resistor when the difference between the voltage on theoutput terminal of the amplifier and the voltage on the input terminalof the amplifier is positive and greater than a predetermined value. Inanother embodiment, the circuit further comprises a second transistorhaving a first terminal, a second terminal, and a control terminal,wherein the first terminal of the second transistor is coupled to theoutput terminal of the amplifier, the control terminal of the secondtransistor is coupled to a second reference voltage, and the firstterminal of the second transistor is coupled to the second resistor. Inanother embodiment, the second reference voltage is configured so thatthe second transistor conducts current between the output terminal ofthe amplifier and the second resistor when the difference between thevoltage on the output terminal of the amplifier and the voltage on theinput terminal of the amplifier is negative and greater than apredetermined value. In yet another embodiment, the circuit furthercomprises at least three resistors coupled in series between a positivesupply voltage and a negative supply voltage, wherein a firstintermittent node between a first two of said at least three resistorsis coupled to the control terminal of the first transistor, and whereina second intermittent node between a second two of said at least threeresistors is coupled to the control terminal of the second transistor.

Alternatively, embodiments of the present invention include an amplifiermethod comprising configuring said amplifier in a first gain, andreconfiguring said amplifier to have a second gain less than the firstgain if the difference between the voltage on an output terminal of saidamplifier and a positive supply voltage for said amplifier is less thana first predetermined value or if the difference between the voltage onthe output terminal of said amplifier and a negative supply voltage isless than a second predetermined value. In one embodiment, reconfiguringcomprises activating a first current path including a resistor betweenthe output terminal of said amplifier and the input terminal of saidamplifier when the voltage difference between the output terminal andinput terminal is positive and greater than a third predetermined value.In one embodiment, reconfiguring comprises activating a second currentpath including the resistor between the output terminal of saidamplifier and the input terminal of said amplifier when the voltagedifference between the output terminal and input terminal is negativeand greater than a fourth predetermined value. In one embodiment, thefirst current path is through a first transistor, and the second currentpath is through a second transistor. In one specific embodiment, thefirst transistor is a PNP transistor and the second transistor is a NPNtransistor. However it is to be understood that other transistors couldbe used.

The following detailed description and accompanying drawings provide abetter understanding of the nature and advantages of the presentinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an example of a prior art amplifier.

FIG. 2 is an example of a circuit according to one embodiment of thepresent invention.

FIG. 3 is an example of a circuit according to another embodiment of thepresent invention.

FIG. 4 is a specific example of a circuit according to one embodiment ofthe present invention.

DETAILED DESCRIPTION

Described herein are variable feedback circuits and methods. In thefollowing description, for purposes of explanation, numerous examplesand specific details are set forth in order to provide a thoroughunderstanding of the present invention. It will be evident, however, toone skilled in the art that the present invention as defined by theclaims may include some or all of the features in these examples aloneor in combination with other features described below, and may furtherinclude obvious modifications and equivalents of the features andconcepts described herein.

FIG. 2 is an example of a circuit according to one embodiment of thepresent invention. Circuit 200 includes an amplifier 201 configured asan inverting amplifier. The inverting amplifier is used throughout thisspecification to illustrate the drawbacks of the prior art andadvantages of the different embodiments of the present invention.However, it is to be understood that other single-ended or differentialfeedback amplifier configurations could use the techniques describedherein. Amplifier 201 includes first and second input terminals (e.g.,positive and negative inputs) and an output terminal. In this example,amplifier 201 includes a first resistor R1 coupled between an inputsignal Vin and the first negative input terminal. A second resistor R2is coupled between the output terminal of the amplifier and the firstinput terminal. The second positive input terminal is coupled to ground.A variable feedback circuit 210 is coupled between the input terminaland the output terminal. As mentioned above, the gain of this stage isgiven by the following equation:Vout/Vin=−R2/R1.In this example, amplifier 201 is powered by a positive supply voltageVdd and a negative supply voltage, which in this example is ground. Itis to be understood that a variety of voltages could be used as thepositive and negative supplies.

Variable feedback circuit operates to reduce the gain of the circuitwhen the voltage on the output terminal is within some predeterminedrange of the positive or negative supply voltages. For instance, if thedifference between the voltage on the amplifier output terminal and thepositive supply voltage is less than a predetermined value, then thevariable feedback circuit reconfigures the amplifier to have a secondgain less than the first gain. Accordingly, as an input signal isamplified and causes the output voltage to increase in the positivedirection close to the upper rail, the gain may be automatically reducedso that the output will not clip. Similarly, if the difference betweenthe voltage on the output terminal and the negative supply voltage isless than a predetermined value, then the variable feedback circuitreconfigures the amplifier to have a gain less than the first gain.Accordingly, as an input signal is amplified and causes the outputvoltage to decrease close to the lower rail (or move generally in thenegative direction close to a negative rail), then the gain may beautomatically reduced so that the output will not clip. The gain of thereconfigured circuit may be the same or different when near the positiveor negative supply voltages.

Accordingly, circuit 200 is also an example of an amplifier methodcomprising configuring said amplifier in a first gain, and reconfiguringsaid amplifier to have a second gain less than the first gain if thedifference between the voltage on an output terminal of said amplifierand a positive supply voltage for said amplifier is less than a firstpredetermined value or if the difference between the voltage on theoutput terminal of said amplifier and a negative supply voltage is lessthan a second predetermined value. The predetermined values may beparticular voltages that are selected as a matter of design choice. Insome embodiments, the first and second predetermined values may bedesigned to be approximately the same.

In one embodiment, variable feedback circuit 210 may include a pluralityof current paths. Accordingly, reconfiguring the circuit may includeactivating a first current path between the output terminal of saidamplifier and the input terminal of said amplifier when the voltagedifference between the output terminal and input terminal is positiveand greater than another predetermined value. Since the current path isbetween the output terminal and the input terminal of the amplifier,such current path is another feedback path. If a resistor is placed inthis feedback path, for example, the gain of the circuit may bemodified. In one embodiment, reconfiguring comprises activating a secondcurrent path including a resistor between the output terminal of saidamplifier and the input terminal of said amplifier when the voltagedifference between the output terminal and input terminal is negativeand greater than a fourth predetermined value.

FIG. 3 is an example of a circuit according to another embodiment of thepresent invention. In one embodiment, the variable feedback circuitcomprises a comparison circuit 311 and feedback circuit 312. Comparisoncircuit 311 and feedback circuit 312 are coupled between the outputterminal and input terminal of amplifier 301. Comparison circuit 311 mayreceive a reference voltage Vref. Feedback circuit 312 may include oneor more current feedback paths that are activated by comparison circuit311. For example, comparison circuit 311 may compare the voltage on theoutput terminal to the voltage on the input terminal and activate acurrent path between the output terminal and the input terminal when thevoltage difference between the output terminal and input terminal ispositive and greater than a predetermined value. In another embodiment,comparison circuit 311 may compare the voltage on the output terminal tothe voltage on the input terminal and activate a current path betweenthe output terminal and the input terminal when the voltage differencebetween the output terminal and input terminal is negative and greaterthan a predetermined value. In one particular embodiment, the feedbackcircuit includes a resistor that is used to change the gain of thecircuit.

FIG. 4 is a specific example of a circuit 400 according to oneembodiment of the present invention. In this example, circuit 400includes an amplifier 401 including a resistor R1 having a firstterminal coupled to the input terminal (here, the negative input) and asecond terminal coupled to an input signal, Vin. Another resistor R2 iscoupled between the amplifier input terminal and output terminal.Amplifier 401 further includes a second terminal (here, the positiveinput) coupled to a reference voltage, which in this example is ground.Amplifier 401 is powered by positive supply voltage Vdd and a negativesupply voltage, which in this case is ground.

In this example, a variable feedback circuit is coupled to oppositeterminals of R2. The circuit shown illustrates a means for coupling aresistor between the output terminal and input terminal of an amplifierif the voltage difference between the output terminal and input terminalis positive and greater than a first predetermined value, or if thevoltage difference between the output terminal and input terminal isnegative and greater than a second predetermined value. In this example,the variable feedback circuit comprises a transistor Q1 having a firstterminal coupled to the output terminal of amplifier 401, a secondterminal coupled to the input terminal of amplifier 401 through aresistor R3, and a control terminal coupled to a first reference voltagethrough a resistor R4. The circuit further includes a second transistorQ2 having a first terminal coupled to the output terminal of amplifier401, a second terminal coupled to the input terminal through resistorR3, and a control terminal coupled to a second reference voltage throughresistor R5. In this example, transistor Q1 is a PNP bipolar transistorsand transistor Q2 is an NPN bipolar transistor. However, it is to beunderstood that other devices such as MOS, for example, could be used.

The reference voltages for Q1 and Q2 are generated by series resistorsR6, R7, and R8, which are coupled between positive supply voltage, Vdd,and negative supply voltage (here, ground). A first intermittent nodebetween resistors R6 and R7 is coupled to the control terminaltransistor Q1. Similarly, a second intermittent node between resistorsR7 and R8 is coupled to the control terminal of transistor Q2. Thesereference voltages set the trip points for the comparison function ofthe circuit. Capacitors C1 and C2 may be included between the emittersof Q1 and Q2 to shunt high frequencies.

In one embodiment, the first reference voltage is configured so thattransistor Q1 conducts current between the output terminal of theamplifier and resistor R3 when the difference between the voltage on theoutput terminal of the amplifier and the voltage on the input terminalof the amplifier is positive and greater than a predetermined value. Forexample, if Vdd is 5 volts, and the voltage at the intermittent nodebetween R6 and R7 is 80% of Vdd (i.e., 4 volts), then as the voltage onthe output terminal of amplifier 401 increases above 4 volts, Q1 willstart to turn on. In particular, as Vout approaches 5 volts, and thebase-emitter voltage increases above the turn-on voltage of Q1, Q1 willconduct current from the output of amplifier 401 through resistor R3 andback to the input of amplifier 401. Accordingly, R3 will now be inparallel with R2, and the gain of the circuit will be reduced. Thevalues of R1, R2, and R3 can be selected by design choice to activatethe variable feedback circuit at any desired difference between theoutput and the supply. A variety of resistor values may be selected forR1, R2, and R3 resulting in a variety of gains and gain changes when thevariable feedback circuit is active or inactive. Similarly, the secondreference voltage is configured so that transistor conducts Q2 currentbetween the output terminal of the amplifier and resistor R3 when thedifference between the voltage on the output terminal of the amplifierand the voltage on the input terminal of the amplifier is negative andgreater than a predetermined value.

The above description illustrates various embodiments of the presentinvention along with examples of how aspects of the present inventionmay be implemented. The above examples and embodiments should not bedeemed to be the only embodiments, and are presented to illustrate theflexibility and advantages of the present invention as defined by thefollowing claims. Based on the above disclosure and the followingclaims, other arrangements, embodiments, implementations and equivalentswill be evident to those skilled in the art and may be employed withoutdeparting from the spirit and scope of the invention as defined by theclaims. The terms and expressions that have been employed here are usedto describe the various embodiments and examples. These terms andexpressions are not to be construed as excluding equivalents of thefeatures shown and described, or portions thereof, it being recognizedthat various modifications are possible within the scope of the appendedclaims.

1. A circuit comprising: an amplifier having an input terminal, anoutput terminal, a positive supply voltage, and a negative supplyvoltage, the amplifier configured to have a first gain; and a variablefeedback circuit coupled between the input terminal and the outputterminal, wherein if the difference between the voltage on the outputterminal and the positive supply voltage is less than a predeterminedvalue, then the variable feedback circuit reconfigures the amplifier tohave a second gain less than the first gain, and if the differencebetween the voltage on the output terminal and the negative supplyvoltage is less than a predetermined value, then the variable feedbackcircuit reconfigures the amplifier to have a third gain less than thefirst gain.
 2. The circuit of claim 1 wherein the variable feedbackcircuit comprises a comparison circuit.
 3. The circuit of claim 2wherein the comparison circuit compares the voltage on the outputterminal to the voltage on the input terminal and activates a firstcurrent path between the output terminal and the input terminal when thevoltage difference between the output terminal and input terminal ispositive and greater than a predetermined value.
 4. The circuit of claim2 wherein the comparison circuit compares the voltage on the outputterminal to the voltage on the input terminal and activates a secondcurrent path between the output terminal and the input terminal when thevoltage difference between the output terminal and input terminal isnegative and greater than a predetermined value.
 5. The circuit of claim1 wherein the variable feedback circuit comprises: a feedback circuit.6. The circuit of claim 5 wherein the variable feedback circuitcomprises a resistor and at least one transistor.
 7. The circuit ofclaim 1 wherein the variable feedback circuit comprises: a firsttransistor having a first terminal coupled to the output terminal, asecond terminal coupled to the input terminal through a first resistor,and a control terminal coupled to a first reference voltage; and asecond transistor having a first terminal coupled to the outputterminal, a second terminal coupled to the input terminal through thefirst resistor, and a control terminal coupled to a second referencevoltage.
 8. The circuit of claim 7 wherein the first and secondtransistors are bipolar transistors.
 9. The circuit of claim 7 whereinthe amplifier includes a third resistor having a first terminal coupledto the input terminal and a second terminal coupled to an input signal,and a fourth resistor coupled between the amplifier input terminal andoutput terminal, wherein the amplifier further comprises a secondterminal coupled to ground, and wherein the first transistor is coupledto a first intermittent node in a plurality of series connectedresistors coupled between the positive supply voltage and the negativesupply voltage, and the second transistor is coupled to a secondintermittent node in said plurality of series connected resistors. 10.The circuit of claim 1 wherein the variable feedback circuit comprisesmeans for coupling a resistor between the output terminal and inputterminal if the voltage difference between the output terminal and inputterminal is positive and greater than a first predetermined value or ifthe voltage difference between the output terminal and input terminal isnegative and greater than a second predetermined value.
 11. A circuitcomprising: an amplifier having a first and second input terminals, anoutput terminal, a positive supply voltage, and a negative supplyvoltage; a first resistor coupled between the output terminal and thefirst input terminal; a first transistor having a first terminal, asecond terminal, and a control terminal, wherein the first terminal ofthe first transistor is coupled to the outPut terminal of the amplifierand the control terminal is coupled to a first reference voltage; and asecond resistor coupled between the second terminal of the firsttransistor and the first input terminal of the amplifier, wherein thefirst reference voltage is configured so that the first transistorconducts current between the output terminal of the amplifier and thesecond resistor when the difference between the voltage on the outputterminal of the amplifier and the voltage on the input terminal of theamplifier is positive and greater than a predetermined value.
 12. Acircuit comprising: an amplifier having a first and second inputterminals, an output terminal, a positive supply voltage, and a negativesupply voltage; a first resistor coupled between the output terminal andthe first input terminal; a first transistor having a first terminal asecond terminal, and a control terminal, wherein the first terminal ofthe first transistor is coupled to the output terminal of the amplifierand the control terminal is coupled to a first reference voltage; asecond resistor coupled between the second terminal of the firsttransistor and the first input terminal of the amplifier; and a secondtransistor having a first terminal, a second terminal, and a controlterminal, wherein the first terminal of the second transistor is coupledto the output terminal of the amplifier, the control terminal of thesecond transistor is coupled to a second reference voltage, and thesecond terminal of the second transistor is coupled to the secondresistor, wherein the second reference voltage is configured so that thesecond transistor conducts current between the output terminal of theamplifier and the second resistor when the difference between thevoltage on the output terminal of the amplifier and the voltage on theinput terminal of the amplifier is negative and greater than apredetermined value.
 13. A circuit comprising: an amplifier having anfirst and second input terminals, an output terminal, a positive supplyvoltage, and a negative supply voltage; a first resistor coupled betweenthe output terminal and the first input terminal; a first transistorhaving a first terminal, a second terminal, and a control terminal,wherein the first terminal of the first transistor is coupled to theoutput terminal of the amplifier and the control terminal is coupled toa first reference voltage; a second resistor coupled between the secondterminal of the first transistor and the first input terminal of theamplifier; and at least three resistors coupled in series between apositive supply voltage and a negative supply voltage, wherein a firstintermittent node between a first two of said at least three resistorsis coupled to the control terminal of the first transistor, and whereina second intermittent node between a second two of said at least threeresistors is coupled to the control terminal of the second transistor.14. An amplifier method comprising: configuring said amplifier in afirst gain; and reconfiguring said amplifier to have a second gain lessthan the first gain if the difference between the voltage on an outputterminal of said amplifier and a positive supply voltage for saidamplifier is less than a first predetermined value or if the differencebetween the voltage on the output terminal of said amplifier and anegative supply voltage is less than a second predetermined value. 15.The method of claim 14 wherein reconfiguring comprises activating afirst current path including a resistor between the output terminal ofsaid amplifier and the input terminal of said amplifier when the voltagedifference between the output terminal and input terminal is positiveand greater than a third predetermined value.
 16. The method of claim 15wherein reconfiguring comprises activating a second current pathincluding the resistor between the output terminal of said amplifier andthe input terminal of said amplifier when the voltage difference betweenthe output terminal and input terminal is negative and greater than afourth predetermined value.
 17. The method of claim 16 wherein the firstcurrent path is through a first transistor, and wherein the secondcurrent path is through a second transistor.
 18. The method of claim 17wherein first transistor is a PNP transistor and the second transistoris a NPN transistor.